Hydraulic nuts are well known and have been used throughout the industry for many decades. Prior art includes individual nuts that are made up of an inner body that is threaded on to the stud to be tightened, an outer body that acts as a piston to generate an axial tension load to clamp the work pieces being joined and a locking collar to mechanically maintain the axial load generated by the hydraulic pressure in the annular piston created between the inner and outer bodies. The gap between the inner and outer bodies needs to be sealed so that hydraulic pressure is generated. This is achieved by a built-in added sealing device.
When multiple studs in a work piece are required to be tensioned, an individual hydraulic nut is installed on to each fastener. Each nut is then linked together by an external hydraulic hose or rigid tubing to allow a common hydraulic pressure to be transmitted to each nut to generate uniform clamp load.
Generally, the outside diameter of the hydraulic nut is fixed, based on the stud diameter, the hydraulic pressure area, and the cylinder wall thickness to contain the hydraulic pressure. On certain flange assemblies with close bolt pitch, hydraulic nuts cannot be used as this diameter will not allow the hydraulic nut to be fitted without interference with adjacent hydraulic nuts.
Other methods of tensioning groups of studs include using external tensioners. A hydraulic bolt tensioning device consists of a reaction component which is threaded on to a stud and bears against a hydraulic head containing a piston. An aperture at the bottom of the tensioning head provides access to a traditional nut which is turned manually whilst the hydraulic head is under its operating pressure. A tensioner is installed on each stud in the assembly to be tightened. Each tensioner is connected through an external hydraulic hose or rigid tube assembly. Once all studs are suitably tensioned, these tools are removed from the flange assembly to be reused on another connection.
Multi-stud tensioners are used to group individual tensioners into a common segment (see for example the multi-stud tensioners manufactured by Hydratight Sweeney). Multiple tensioning segments are hydraulically connected using an external hose to allow a common load to be applied to each stud in the assembly. An aperture at the bottom of the tensioning segment provides access to a traditional nut which is turned manually, whilst the tensioning segment is under its operating pressure. Once all the studs in the assembly are suitably tensioned the multi-stud tensioning segments are removed from the flange assembly to be used on another connection.
The make up of critical connections such as steam generator manway covers often use external multi-stud segmented tensioners. These covers are installed with traditional studs and nuts. Then multi-stud tensioning segments are lifted into place to tighten the bolts. In the field of nuclear reactors, this slow process will cause additional manrem exposure to field personnel.
The make up of other critical connections such as steam turbines, reactor vessel manways and other flange connections that have tight pitch distances between assembly bolts can prohibit use of hydraulic nuts or external tightening tools such as tensioners.
These tensioning devices are not suitable in applications where assembly times measured in hours and minutes are cost prohibitive.
Also, the makeup of subsea flange assemblies is critical to ensure leak free connections and is a very time consuming process when using the above prior art. Moreover, in deep water flange assemblies, the use of Remote Operated Vehicles (ROV) is common. Traditional hydraulic nuts and tensioners are not generally suitable for use by ROV.
There is therefore a need for a unibody hydraulic nut which obviates the aforementioned problems.